feat: add a durable transcode session store

Transcode state lives only in the process that started ffmpeg, so a pod restart
drops every in-flight HLS stream with no way for a peer to pick it up.

- add ITranscodeSessionStore plus the TranscodeSession and LiveStreamSession records
- add RedisTranscodeSessionStore with TTL leases and an atomic Lua takeover script
- add NullTranscodeSessionStore for single-instance deployments
- pick the store from Jellyfin:TranscodeStore:RedisConnectionString at startup

MediaBrowser.Controller/MediaEncoding/TranscodeStoreOptions.cs

@@ -0,0 +1,19 @@
+namespace MediaBrowser.Controller.MediaEncoding;
+
+/// <summary>
+/// Configuration options for the transcode session store.
+/// </summary>
+public sealed class TranscodeStoreOptions
+{
+    /// <summary>
+    /// Gets or sets the Redis connection string.
+    /// A <c>null</c> or empty value indicates single-instance mode, where
+    /// <see cref="NullTranscodeSessionStore"/> is used instead of a Redis-backed store.
+    /// </summary>
+    public string? RedisConnectionString { get; set; }
+
+    /// <summary>
+    /// Gets or sets the duration in seconds for which a transcoding session lease is valid.
+    /// </summary>
+    public int LeaseDurationSeconds { get; set; } = 30;
+}